Skinny chicks. Is this normal?

Why are my chicks so skinny? Is it normal or do I have an internal infestation of worms or a parasite? I can feel their breastbone and rib cage. They eat like pigs and I feed 18% protein medicated starter, it's the only one thats offered in 50lbs. I've tried egg yolk, but they don't like it. I also give probios and ACV in their water. I don't give them any treats either. They run and fly around the brooder constantly.

I'm stumped to why they are so thin. I also put polyvisol vitamins all last week, but quit that. My bantam got a little pasty butt from the vitamins. Thanks in advance.